The tsunami is an amazingly destructive force. A 30-foot tsunami exerts 49 tons of pressure every square yard. The areas in greatest danger are shallow, sloping coastlines, curved bays, or valley fjords. Tsunamis are usually single, large waves, although there have been several successively larger waves noted in areas like Hawaii.
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/tsunamis/impact.shtml
Tsunamis usually occur in the Pacific Ocean, and there has been at least one a year since 1800. A destructive tsunami can be expected every ten years.